re: identifying the warlock models: if they ahve swords or spears, they're warlocks. If they have a staff, they're a spiritseer. There's a new plastic spiritseer with a staff and a dagger
Farseers work very well alongside Warlocks, as Warlocks give access to the Runes of Battle spell list and also let you use the Seer Council stratagem, which significantly boosts both of them. If you're allowed to go up to a Battalion, I highly recommend it.
Regarding troops: If you're staying as a patrol, I highly recommend merging the two guardian squads. Why? Because certain stratagems (Celestial Shield, for example), and certain spells (Guide, Fortune, Protect, etc) buff Guardians on a per-unit basis. This means that a 20-elf guardian squad benefits twice as hard from eldar buffs. If you're hopping up to a Battalion, I'd consider adding in a 5-elf Dire Avenger squad and merging the Guardians. I'd run the 20-elf Guardian squad whether you use webway strike or not, since you can buff them up more easily to keep them durable. I like using webway strike with a 20-elf guardian squad.
Striking Scorpions are a solid disruption unit, just bear in mind that unless you do something to boost their charging (biel-tan stratagem, saim-hann craftworld trait, etc) they're not likely to stick a charge when they arrive onto the table. I like to drop them into cover in case they miss their charge. These guys are not hero-killers or anything, but are great for disrupting enemy shooting units that do not want to be in melee.
Swooping hawks are great. Make sure they shoot at a Doomed (and Jinxed, perhaps) target unless they're shooting at something very light, like Guardsmen.
Windriders - I'd try to give these guys all the same wargear, that way it's easy to tell what range to put them at.
War Walkers - I'd consider merging these into one squad for reasons of buffing, but running them separately can be fine as well. Personally, I think that Shuriken Cannon War Walkers are pretty decent and are also cheap, and i don't normally put more expensive guns on them. I consider War Walkers to be a bit flimsy in 8e, even compared to their usual flimsiness, so I try to keep them inexpensive and disposable
regarding your unused minis:
Banshees are a lot like Striking Scorpions in that they mostly want to focus on annoying the enemy, getting the enemy's gunline into unwanted melee, etc. They don't have a built in delivery mechanism, which is a problem, but they can charge a heavily armed opponent without worrying about overwatch, which gives you more options. It's fine to leave them out in a smaller list like this.
Warp Spiders are in a tough place this edition. They're harder to shoot than most of our infantry, but their price is just a bit on the high side atm compared to things like windriders, swooping hawks, and so on. I don't think they're bad, but our other stuff in their weight class is a bit better.
Wave Serpents are great, I love using them to deliver fire dragons, wraithguard, etc, and after they drop off their delivery they are respectable battle tanks. I almost always prefer Wave Serpent to a Falcon.
I'd definitely consider bringing a cheap Wraithlord - if you have the old metal "eldar dreadnought" style Wraithlord, you can just run it with nothing up top, as a mega cheap wraithlord with just his fists and 2 shuriken catapults. This is actually very cheap, very durable, and capable of taking on lots of enemies just by Advancing up the field and eventually getting a charge in.
Fire Prisms usually operate a lot better in pairs or groups of 3 due to the Linked Fire stratagem, but they're not that bad on their own. I wouldn't take a single one though.
Regarding new minis: A Warlock or Farseer skyrunner is a fine choice, but as you already have both on foot, it's not necessary. The models you have are sufficient for a wide variety of 750-1,000 pt battles already, and it looks like you're on the right track for list building. In terms of areas of weakness in your collection, the biggest that jumps out at me is anti-tank. only your heavy support units seem to have the capability, but you have weaknesses:
- 3 war walkers with BLs is actually pretty decent if you buff them up, though they are flimsy and will get targeted down
- you only have one fire prism, which means you can't use the Linked Fire stratagem to boost its firepower
- your Wraithlord is an old metal wraithlord, so he can't take a sword to make his melee threatening to T8 targets
- Falcons are a bit on the pricey side for what they do this edition, though Falcon with BL is a reasonable threat
- Wave Serpent with Bright Lances is actually solid, though usually he will have other jobs besides just shooting in the first half of the battle, which reduces his effectiveness
If you do want to make a purchase, I'd consider something that can put out high strength firepower in a cost-effective way, and either take a beating in return, or at the very least be able to provide that firepower without fail. We have a lot of options for this, actually. In terms of decent looking plastic kits:
Wraithguard with wraithcannons are decent at this if they have a Wave Serpent to deliver them. Even armed with D-Scythes they're a threat to most vehicles.
Crimson Hunters are some of our best tank killers if you don't mind taking a flyer, and the mini itself is very handsome.
Hemlock Wraithfighter can also take down tough targets with its powerful Heavy D-Scythe.
If you don't mind working with resin:
Fire Dragons have excellent anti-tank firepower for the cost, though they need a delivery mechanism (a wave serpent is good for this) and often die after shooting
Dark Reapers can provide great long ranged firepower to bust tanks
Buying an additional fire prism, or one of the new wraithlords with the ability to take a sword, may also work well for you.
There are also other ways to crack open rhinos, dreadnoughts, and leman russ battle tanks available to eldar. You could take a lot of psykers, or use Shining Spears with Doom, and so on, but these are the things that come to mind for me.